The present invention relates to a tire inflation monitor for vehicles. More specifically, the present invention relates to a receiver for receiving tire inflation pressure data transmitted by a wireless transmitter located in a tire.
Wireless tire inflation pressure monitors have been proposed so that the tire inflation pressures of tires of a vehicle can be checked in the passenger compartment. The monitor includes transmitters attached to the wheels and a receiver located in the body of the vehicle. Each transmitter measures a tire inflation pressure of a corresponding tire and performs wireless transmission of data including the measured inflation pressure data. The receiver receives the data from each transmitter and displays information concerning tire inflation pressure of each tire on a display, which is located, for example, in front of the driver's seat.
A typical vehicle having four tires includes four transmitters corresponding respectively to the tires. The output strength of radio waves from each transmitter is limited by local communications laws and cannot be greatly increased. The receiver has four reception antennas that respectively correspond to the four transmitters, to ensure reception of weak radio waves from the transmitters. The reception antennas are located in the vicinity of the tires, which have the built-in transmitters, for example, in or near the fenders (cf. Japanese Unexamined Utility Model Publication No. 2-74204). The receiver receives the weak radio waves from the transmitters through the corresponding reception antennas.
A receiver unit is located in the passenger compartment or the trunk. The reception antennas are located apart from the receiver unit and are connected to the receiver unit through cables. Therefore, the wiring between the receiver unit and the reception antennas is complicated. This complicates the entire system and the cost of the monitor.
The reception antennas described in the above Publication No. 2-74204 are arcuate wire antennas, which extend along the fenders. Each wire antenna induces voltage mainly by an electric field component of radio waves. However, most vehicle bodies have metal fenders, which are likely to influence the electric field component of radio waves. The electric field component of radio waves attenuates in the vicinity of metal materials. Therefore, the wire antennas attached to the fenders cannot efficiently receive radio waves from the transmitters.
To improve the efficiency of receiving radio waves, each wire antenna must be separated from the metal surface of the corresponding fender as much as possible. However, if the distance between a wire antenna and the metal surface is varied even slightly, the performance of the wire antenna varies greatly. This makes it difficult to maintain the quality of the monitor. Also, when one of the wire antennas is separated from the metal surface of the corresponding fender, the wire antenna may interfere with the corresponding tire.
The length of each wire antenna may be increased to improve the efficiency of receiving radio waves. However, his increases the size of the wire antennas and is not desirable.